Follicular thyroid cancers comprise roughly 10 percent of all thyroid cancers. They are often difficult to distinguish from benign follicular adenomas (see Chapter 8), and we strongly suggest that a second pathologist verify the diagnosis by analyzing the thyroid gland removed during surgery. These types of thyroid cancer are more likely to spread via the bloodstream to distant sites, such as the lung, bones, and liver. They tend to be more aggressive than typical papillary thyroid cancers, but less aggressive than tall cell variant papillary cancers. Hurthle cell thyroid cancers (also known as oxyphilic variant follicular cancers), like their papillary counterparts, may lose the ability to take up radioactive iodine, making them difficult to treat if they recur after surgery.
A much less common type of follicular thyroid cancer is known as insular thyroid cancer. More than a decade ago, insular carcinomas were thought to be types of anaplas-tic thyroid cancer, suggesting that they could not be treated with radioactive iodine and were likely to be lethal. Since then, physicians have learned that insular carcinomas are a subtype of follicular thyroid cancer that sometimes takes up radioactive iodine. They are usually much more aggressive than follicular cancers and require special attention.
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